Television receiving apparatus



Patented July 4, 1939 UNITED STATES PATENT OFFICE Merson Spratt,

Slough, England Application July 30, 1937, Serial No. 156,502 In Great Britain August 5, 1936 Claims.

This invention relates to television receiving apparatus.

In television receiving apparatus it is frequently desirable to locate a loudspeaker in 5, proximity to a cathode ray tube, but with loudspeakers employing permanent or electro-magnet systems of the type now common in use, a diiiiculty occurs owing to the magnetic field of the loudspeaker magnet when in operation, causing lil a permanent deflection of the electron beam of the cathode ray tube. This deflection of the beam will be super-imposed on the excursions of the beam due to the scanning system resulting in the reproduced picture being bodily displaced, and furthermore, since the leakage field from the magnet system is unlikely to be uniform over the space traversed by the beam, even when the cathode ray tube is encased in a suitable shielding box, the reproduced picture Will probably not only be displaced but also distorted.

In addition, the effect of the field of the loudspeaker upon the cathode ray tube will vary according to the relative positions of the loudspeaker and the cathode ray tube. In the manufacture of receiving apparatus it is of course not desirable that the layout of the receiving apparatus be dependent upon the relative positions of the loudspeaker and the cathode ray tube.

It is the chief object of the present invention to provide an improved arrangement for the purpose of neutralizing the leakage field from the magnet of the loudspeaker with a view to preventing displacement or distortion of the picture reproduced by the cathode ray tube and in which the field from the loudspeaker is substantially neutralized despite the relative positions of the loudspeaker and the cathode ray tube.

According to the invention a television receiving apparatus is provided including a cathode ray tube and loudspeaker having a permanent or an electro-magnet, and wherein, in order to neutralize the leakage field from the magnet of the loudspeaker an electro-magnetic neutralizing coil is provided mounted in close proximity to the loudspeaker and in such a position as effectively to neutralize the leakage eld from the loudspeaker. In the preferred form of the invention the electromagnetic neutralizing system is mounted directly on the rear surface of the loudspeaker magnet and comprises a core upon which a plurality of turns of wire is Wound and on each side of the coil cheeks or members of magnetic material are mounted, the cheeks being of dimensions comparable with that of the (Cl. ITS- 5.8)

magnet system of the loudspeaker. The number of turns of wire in the neutralizing system will be suiiicient so as to cause eilective neutralization of the leakage field from the loudspeaker magnet, and since the leakage field will thus be effectively neutralized at its source the relative positions of the loudspeaker and the cathode ray tube in the receiving apparatus will not be critical.

In order that the said invention may be clearly understood and readily carried into effect the same will now be more fully described with reference to the accompanying drawing in which:

Figure 1 is a plan View of a part of a loudspeaker electromagnet with a neutralizing system mounted thereon, v

Figure 2 is a rear view of Figure 1, and

Figure 3 is a cross-section along the lines A-A of Figure 2.

As shown in the drawing, an electromagnet is employed in the loudspeaker comprising the usual energizing windings 4 surrounding a center polepiece 5 carried by a yoke 6. The neutralizing system comprises a core 'l upon which is wound a plurality of turns of wire 8 and on each side of the core 8 cheeks or members 9 and I 0 of magnetic material are mounted, the cheeks being of dimensions comparable with that of the electromagnet of the loudspeaker. That is to say, the length and Width or diagonals of the cheeks should be at least approximately equal to the maximum dimension of the rear end of the magnet system. The neutralizing system is mounted directly on the rear surface of the loudspeaker magnet, the core 'l being arranged coaxially with the center pole-piece 5, the cheeks 9 and I0 being disposed at right angles to the core 1 and center pole-piece 5. The two cheeks 9 and I0 may be made of rectangular plates of a suitable material, such as mild steel, and as shown are clamped in position against the core 8 by screws Il entering. the spacing members I2 at the corners of the cheeks, sheets of suitable insulating material I3 being interposed between the turns 8 and the adjacent surfaces of the cheeks. The neutralizing system is, as stated above, secured to the rear of the loudspeaker magnet and for this purpose it is maintained in position by a screw I4 passing through the core 1 and the cheeks. the screw being threaded into a nipple l5 in turn threaded into the center pole-piece 5. The nipple l5 is flanged, as shown, in order to space the neutralizing system from the rear of the yoke B. The cheeks nearest the loudspeaker magnet tend to symmetrize the leakage eld from the loud- Cil speaker magnet; the cheek remote from the magnet converts the eld arising from the compensating Winding from What would otherwise approximate to a point source into a magnetic field of reasonably constant density over a. wide area. As a result there is greater likelihood of complete neutralization over the Whole area required at one value of compensating iield. The size and shape of the cheeks can best be determined empirically.

In one specic example as applied to a loudspeaker having an electromagnet of the claw type and placed in front of a 12 cathode ray tube encased in the usual iron box and separated therefrom by about l, the leakage field from the loudspeaker was found to cause deflection of V the electron beam by approximately 1. The loudspeaker magnet had a claw the base of which was approximately 4 long and 1% Wide and thick, a center pole of about 3A diameter, with a eld coil of approximately 21/4 diameter surrounding the center pole. It was found that this deflection was overcome by emn ploying a neutralizing electromagnet system comprising 1,80() turns of 32 S. W. G. copper Wire Wound on a 3A diameter mild steel rod 1/ long. Two end cheeks about 4" square and made of thick mild steel were secured against the sides of the coil and the current passing through the loudspeaker magnet coil of about Si) milli amperes was employed to excite the neutralizing electromagnet which Was sufcient to prevent deflection of the reproduced picture. 1n the claw type of electromagnet the loudspeaker magnet is not symmetrical radially, and hence it is desirable to employ a cheek interposed oetvveen the neutralizing coil and the loudspeaker magnet for the purpose above-mentioned. The invention may equally be employed where the loudspeaker employs a permanent magnet.

In some cases the neutralizing magnet system may be provided with an adjustable shunt coil to obtain a control of the compensation, or alternatively, over-compensation may be introduced, and so deflection other than that due to the speaker eld may be neutralized.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, We .declare that what We claim is:

l. A loudspeaking apparatus comprising means for establishing a driving field, said means including a magnetic core, means for utilizing said field, at least one cheek member mounted adjacent said magnetic core on the side thereof remote from said eld utilizing means, and a coil member mounted adjacent said cheek member for neutralizing external iields from said core member.

2. A loudspeaking apparatus comprising means for establishing a driving iield, said means including a magnetic core, means for utilizing said eld, a pair of cheek members mounted adjacent said magnetic core on the side thereof remote from said field utilizing means, and a coil member mounted adjacent said cheek member for neutralizing external elds from said core member.

3. A loudspeaking apparatus comprising means for establishing a driving field, said means including a magnetic core, means for utilizing said eld, at least one cheek member mounted adjacent said magnetic core on the side thereof remote from said eld utilizing means, said cheek member having an area at least as great as the side of said core to which it is adjacent, and a coil member mounted adjacent said cheek member for neutralizing external fields from said core member.

4. A loudspeaking apparatus comprising means for establishing a driving field, said means including a magnetic core, means for utilizing said eld, a pair of cheek members mounted adjacent said magnetic core on the side thereof remote from said field utilizing means, said cheek members having an area at least as great as the side of said core to which they are adjacent, and a coil member mounted adjacent said cheek mem ber for neutralizing external fields from said core member.

5. A television receiving apparatus including a cathode ray tube and a loudspeaker, the former for reproducing the television image and the latter for reproducing the sound, said loudspeaker including a magnetic yoke member adapted to generate an energizing eld, means for utilizing said energizing field to reproduce sound signals, and a cheek member mounted on said magnetic core member on the side remote from said iield utilizing means, and a magnetic coil mounted on said cheek member for neutralizing stray fields which normally would spuriously disturb the beam in said cathode ray tube.

JOHN EDWARD MEAD. HECTOR GORDON MERSON SPR/ATT. 

